An X-ray tube, an example of an X-ray generating device, is a vacuum device in which an electron beam from a cathode is accelerated by an electric field and is made to hit a target to generate X-rays by the impact. Among X-ray tubes, those in which an electron beam is made to hit one of the surfaces of a target and X-rays are radiated from the other surface of the target are called transmission type X-ray tubes. Transmission type X-ray tubes are used in non-destructive inspections, thickness measurements, X-ray analysis, etc. For example, to perform non-destructive inspection of an electronic part or other compact, high-density item, a transmission type X-ray tube is required to have a micro focusing function.